Security is an important concern in the realm of documents and digital production and/or reproduction of same. Known digital image printing/copying systems produce documents of such high quality that a need has been identified to prevent effective printing/copying of certain documents such as high-value printed items including tickets, financial instruments, security passes, and the like. Known techniques include printing the original document in a manner such that it includes a digital “watermark” using only conventional paper and toner or ink. A digital watermark is defined as information, for example one or more letters, words, symbols or patterns, that is at least partially (preferably fully or at least substantially) hidden in a printed image under normal viewing conditions but that is more clearly discernable under certain specialized viewing conditions. Unauthorized reproduction of documents including such digital watermarks typical degrades or obscures the digital watermark, which can aid in detection of counterfeit documents.
A fluorescence mark is one example of a known digital watermark. Methods and systems are known for including fluorescence marks in printed documents using conventional papers (e.g., ordinary “copy paper” or “printer paper”) and ordinary inks/toners (e.g., CMYK ink/toner), specifically by using metameric colorant mixtures. Under visible lighting conditions (e.g., electromagnetic radiation wavelengths of about 400-700 nanometers (nm), the different colorant mixtures that are printed on respective adjacent portions of the paper together define an overall printed document region that appears substantially uniform in color. Under ultraviolet (UV) lighting (e.g., electromagnetic radiation wavelengths shorter than about 400 nm), these different colorant mixtures exhibit different UV absorption and, thus, different suppression of UV fluorescence of the optical brightening agents used in conventional printing/copying papers such that the region printed with the colorant mixture that suppresses less of the substrate fluorescence appears as a lighter/brighter region while the adjacent area printed with the colorant mixture that strongly suppresses substrate fluorescence appears as a darker region. These contrast variations under UV lighting are used to create watermark patterns, e.g., numbers, letters, symbols, shapes.
An example of this is shown in FIG. 1, wherein a colorant mixture “B” is selected and applied to patch area BP which, in this example, is shaped as the alphanumeric symbol “0”. Further, a colorant mixture “A” is selected and applied to patch area AP arranged here in substantially close spatial proximity to patch area BP, and thereby providing a background around patch area BP. Both colorant mixture A and colorant mixture B are comprised of a suitably selected colorant or colorant mixtures, but colorant mixtures A and B are different mixtures. Each colorant mixture A or B may be, for example, either a single CMYK colorant or any mixture of CMYK colorants. In the illustrated example, colorant mixture A will be selected so as to provide higher UV absorption (greater substrate fluorescence suppression) than that selected for colorant mixture B. The colorant mixtures A and B will also be selected to match each other closely in their average color and luminance when viewed under visible light conditions. As shown at UV in FIG. 1, under UV lighting conditions, patch BP will appear brighter as compared to patch AP, due to the relatively limited suppression of the fluorescence of the optical brightening agents in the paper substrate as compared to the patch AP, thus forming a watermark W1. In contrast, under visible light conditions as shown at VIS, patches AP,BP are at least substantially indistinguishable. By way of example, an approximate 50% gray color may be realized with a halftone of black (K) colorant only and used for colorant mixture B to print patch BP. This may then be color-matched against a colorant mixture comprising yellow (Y), cyan (C), and magenta (M) that yield a similar approximate 50% gray color, A which is used to print the patch AP. In general, colorant mixture A will cover more of the paper, thus providing much higher suppression of native substrate fluorescence as compared to the patch BP, so that under UV lighting conditions, the patch BP will be readily apparent as a watermark W1. The two colorant mixtures will appear quite nearly identical “gray” under normal visible light viewing as shown at VIS in FIG. 1. Thus, when a document including such a fluorescence mark is subjected to UV illumination, the watermark W1 is revealed. A printed “look-alike” document or mere photocopy will not properly reproduce the watermark. Additional details and variations relating to fluorescence marks are disclosed in U.S. patent application Ser. No. 11/382,897 filed May 11, 2006 in the name of Raja Bala and Reiner Eschbach and entitled “Substrate Fluorescence Mask for Embedding Information in Printed Documents” and U.S. patent application Ser. No. 11/382,869 filed May 11, 2006 in the name of Raja Bala and Reiner Eschbach and entitled “Substrate Fluorescence Pattern Mask for Embedding Information in Printed Documents” and the disclosures of both these applications are hereby expressly incorporated by reference into the present specification.
As noted, a correlation mark is another known watermarking technique. By way of example, as shown in FIG. 2, a checkerboard halftone pattern HP is printed on paper by a regular 45-degree cluster halftone screen and has 13 columns of halftone cells. Another checkerboard halftone pattern HP′ covering an equal spatial area overall is printed on paper by a halftone screen having four slightly stretched columns C5-C8 in the middle section, such that the halftone pattern HP′ has only 12 columns over the same spatial width as the pattern HP. The patterns HP and HP′ are perceived as the same gray level to a human observer and the phase shift is substantially hidden under normal image viewing conditions. In the pattern HP′, due to the pitch difference between the regular columns C1-C4,C9-C12 and the slightly stretched columns C5-C8, the phase, which represents the transition between white and black, has changed from “in phase” with the pattern HP, (0 radian phase shift), for columns C1-C4, to “opposite phase” (π radian phase shift) for columns C9-C12, with varying, increasing phase shift for the stretched columns C5-C8. If a transparency key K is printed with the halftone pattern HP and overlaid with the paper-printed halftone pattern HP as shown at K+HP, the result is unchanged relative to the original printed pattern HP because the key K and printed pattern can be registered, solid-on-solid, blank-on-blank. In contrast, when the key K is overlaid with the phase-shifted pattern HP′ as shown at K+HP′, the original printed checkerboard pattern HP′ gradually disappears in proportion to the phase difference between the pattern HP′ and the key K moving from left to right in the image, until the checkerboard pattern HP′ is completely black (or other solid color) where the pattern HP′ and key K are opposite phase. The average reflectance is higher in the registered regions C1-C4 and lower in the partially phase-shifted regions C5-C8 and still lower in the opposite phase regions C9-C12. Therefore, if a portion of an image is encoded with π radian phase-shifted halftone cells and immediately surrounding these π radian phase-shifted halftone cells are halftone cells that are 0 radian phase-shifted, by overlapping a checkerboard pattern transparency key of the same spatial frequency, a high contrast watermark W2 can be defined by the low reflectance areas where the key and halftone pattern are out of phase (conversely, a watermark can be defined by the high reflectance areas where the key and printed document are in phase). Accordingly, a document can be printed in this manner to include a watermark that is completely or at least substantially undetectable until a corresponding transparency key is overlaid to reveal the phase-shifted halftone regions. Correlation marks are disclosed, e.g., in U.S. Pat. No. 6,252,971 to Wang entitled “Digital Watermarking Using Phase-Shifted Stoclustic Screens” which is also hereby expressly incorporated by reference into the present disclosure.
While both fluorescence marks and correlation marks are highly effective, a need has been identified for a watermark that is less apparent under ambient lighting conditions and more apparent during a security check of the document, without requiring use of a separate transparency overlay key. Accordingly, the present development combines principles of fluorescence marks and correlation marks to achieve these goals as disclosed below.